1. Technical Field
Aspects of the present invention relate to an image reading device for reading an image from an original.
2. Related Art
A contact image sensor (CIS) is widely used as a reading device in a facsimile device, a copying device and the like. This type of contact image sensor is configured to execute a reading operation in accordance with a start signal and clock pulses from a control unit as disclosed in Japanese Patent Provisional Publications Nos. 2002-320070, HEI 7-193675, HEI 11-215302, and 2004-64406.
FIG. 7A is a conceptual illustration of a configuration of a contact image sensor unit. As shown in FIG. 7A, the contact image sensor unit includes a light guide 101 provided with light sources 103a and 103b such as an LED at its both ends, and an image sensor 107. The light guide 101 diffuses light from the light sources 103a and 103b in a main scanning direction, which is indicated by an arrow in FIG. 7A, to illuminate an original 105. The image sensor 107 is formed to be elongated in the main scanning direction and receives light reflected from the original 105. The model shown in FIG. 7A supports reading of an A4 size sheet that needs a relatively wide reading area, by providing two light sources at both ends of the light guide 101 to secure a sufficient amount of light for image reading.
FIG. 7B is an explanatory illustration for explaining a drawback that the image reading device shown in FIG. 7A may have. In FIG. 7B, output levels (hereafter, referred to as white levels) of the image sensor 107 obtained when a white object is illuminated are shown. More specifically, a graph C1 represents the white level obtained when the white object is illuminated only by the right side LED 103b, and a graph C2 represents the white level obtained when the white object is illuminated only by the left side LED 103a. As can be seen from graphs C1 and C2, when only one of the LEDs 103a and 103b is activated, the white level gradually decreases as a distance from the LED emitting light increases. Therefore, if one of the LEDs is in an abnormal state, an adequate amount of light for image reading can not be secured.